A 12-year-old boy in a small rural town, Burlington in Colorado, has allegedly shot dead his parents and seriously wounded two of his siblings.

The carnage has caused bewilderment among relatives and neighbours who saw the family as utterly normal, if deeply religious.

Charles Long, 50, and his wife Marilyn, 51, were found shot dead and two of their seven children, Sarah, five, and Ethan, nine, in a critical condition having been slashed with a knife.

Sarah, who was slashed in the neck and back, and Ethan, who was shot and stabbed in the groin, are both recovering in hospital.

The prosecutor, Robert Watson, is now deliberating whether to treat the boy within the adult criminal trial process or as a juvenile.

The decision has huge implications for the child. If he is classified as an adult he could face life in an adult prison. If treated as a juvenile he would be entitled to therapy and rehabilitation and release by the age of 21.

"I am not going to take this decision lightly," Watson told the Denver Post. "This is not the type of thing you expect a 12-year-old boy to do."

Locals say the family appeared outwardly to be normal. All the children, the four eldest of whom have left home, were home schooled and the Longs were pillars of their local evangelical church where the boy often used to volunteer in Bible classes.

The boy's uncle said the tragedy was beyond understanding. "It's a mystery to me. I don't know if anyone can explain it at this point and maybe never will be able to explain it."

He told reporters in Denver that he had not noticed anything untoward about his nephew. "He's a typical kid, probably liked to play video games, that kind of thing. I can't imagine what would have happened, what went through his mind."